Seeing a planarian for the first time, you might not even know what you’re looking at. Brown, black, or white in color, these flatworms are about the size of a toenail clipping and have two light-sensing spots on their triangular-shaped heads that make them look cross-eyed. Their simple appearance, though, belies a surprising ability.

“You can take one of these animals and cut them into 18 fragments,” says Alejandro Sánchez Alvarado, who does this for a living, “and each and every one of those fragments will go on to regenerate a complete animal.”

Sánchez Alvarado is a scientific investigator with the Howard Hughes Medical Institute, who specializes in developmental and cell biology. His laboratory in Kansas City is hosted by the Stowers Institute for Medical Research.

“That would be the equivalent of me cutting my little finger and watching my little finger regenerate me,” he said in an interview with Steve Kraske on KCUR’s Up To Date.

Alejandro Sánchez Alvarado, PhD

Credit Howard Hughes Medical Institute

Sánchez Alvarado has been studying this phenomenon for more than two decades, but planarians have interested scientists for much longer. Charles Darwin observed several different species of the little flatworms on his trips through South America. He wrote in The Voyage of the Beagle that, “having cut one of them transversely into two nearly equal parts, in the course of a fortnight both had the shape of perfect animals.”

Salamanders have a similar ability to re-grow a lopped-off tail, but this unidirectional regeneration is different from the planarian’s bidirectional regeneration powers, in that a new salamander will not spring forth from the severed tail.

The process by which a human regenerates skin cells — something that is absolutely essential to our survival — is called homeostatic regeneration.

“We're not entirely certain how these two things relate to each other,” says Sánchez Alvarado, but it is a process he’s eager to understand.

Planarians are found all over the world — even in the streams of Missouri and Kansas — but Sánchez Alvarado he had to be more particular in his selection of a species to study.

“We actually had to find a very specific species that fulfilled a number of criteria, that would allow us to access the problem of regeneration as rapidly and as simply as possible,” he says. His lab has focused in on Schmidtea mediterranea.

After amputation, planarians can regenerate missing body parts in as little as a week. By manipulating genome pathways, scientists can get them to regenerate a head and a tail (top), no tail (middle) or two tails (bottom).

Credit The Sánchez Laboratory

“What we find is that some of the genes they're using to restore their missing body parts are not unique to planarians,” Sánchez Alvarado says. The trick is figuring out which gene, or combination of genes, provides that regenerative spark.

All this begs the question: Is it possible that humans will someday be able to tap into that mechanism and regenerate an arm or a leg?

“Imagining a human being regenerating a complete arm from scratch after amputation, that sounds a little bit fanciful to me right now,” says Sánchez Alvarado, but “I can envision the possibility of us being able to regenerate specific cell types.”

So, while the prospects for replacing missing or amputated limbs are dim, there is a glimmer of hope, thanks to an unassuming little aquatic invertebrate.

“That’s the beautiful thing about biology,” Sánchez Alvarado says. “All of our cells and all of the genes in those cells are made of the same stuff, the same molecules.”

Alejandro Sánchez Alvarado is a presenter at Kansas City's independently organized TED event, TEDxKC, which starts at 6 p.m. on Friday, August 19, at the Kauffman Center for the Performing Arts. The live and simulcast venues are sold out, but watch party tickets are available. For more information go to TEDxKC.org.

Luke X. Martin is a freelance contributor for KCUR 89.3 and an assistant producer for 'Up To Date.' He can be reached at luke@kcur.org.

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